The present invention is related to a process for preparing a carbon molecular sieve membrane, and in particular for preparing a supported carbon molecular sieve membrane.
Currently, carbon molecular sieve membranes are prepared by subjecting a thermoset polymer to pyrolysis or calcination at a high temperature in an inert gas or vacuum environment, thereby releasing volatile gases, such as H2O, CO, CO2, CH4, HCN, N2 and H2 etc., and forming an amorphous carbon membrane having a pore size of several microns to several angstroms. The pore size is closely related to the material of the polymer and the conditions of pyrolysis. Published researches on the carbon molecular sieve membranes are discussed in the following:
(1) Bird and Trimm used polyfurfuryl alcohol (PFA) to prepare non-supported and supported carbon molecular sieve membranes. Due to the occurrence of shrinkage during pyrolysis, a continuous carbon separation membrane could not be produced [P. L. Trimm et al., Carbon, 21(3), 177, 1983].
(2) Koresh and Soffer have done a very systematic research on carbon molecular sieve membranes [J. E. Koresh and A. Soffer, Sep. Sci. Technol., 18 (1983) 723; J. E. Koresh and A. Soffer, Sep. Sci. Technol., 23 (1987) 973]. A hollow fibrous polymer membrane was calcined at a medium temperature (800-950xc2x0 C.) in nitrogen or an inert gas, thereby forming a carbon molecular sieve membrane. The selectivity of He to O2 was 8, and the selectivity of He to N2 was 20. In particular, the permeance of He reached 3xc3x9710xe2x88x927 mol.mxe2x88x922.sxe2x88x921.Paxe2x88x921, which was tens to hundreds times higher than that of a polymer membrane.
(3) Linkov et al. produced a three-zoned asymmetrical carbon membrane by subjecting a polyacrylonitrile, (PAN)-based hollow fibrous precursor to a thermal oxidation stabilization and a carbonization in an inert atmosphere [V. M. Linkov, R. D. Sanderson and E. P. Jacobs, J. Membrane Sci., 95 (1994) 93]. The intermediate zone had longitudinal voids with a length 5-15 xcexcm and a diameter 3-7 xcexcm. The inner layer had voids with a pore size of 3-5 xcexcm. The outermost layer is a denser layer of 0.1-0.4 xcexcm. Subsequently, a mixture gas of TiCl4 and CH4 was subjected to a gas phase pyrolysis to grow a TiC membrane on said hollow carbon fiber, which was then subjected to a high temperature oxidation in order to reduce the pore size on the outermost layer to less than 90 nm. In 1994, a combined magnetron sputtering and ion beam technique was used to coat a diamond-like carbon (DLC) membrane on the abovementioned fiber. The results indicated that a low sputtering rate could form a continuous membrane which fully covered the original voids. However, this composite membrane (without receiving a further carbonization) still had a Knudsen diffusion mechanism in transporting gas, and not a molecular sieve mechanism.
(4) Yamada et al. produced a carbon molecular sieve *membrane by subjecting a polyimide (PI) to a carbonization [Y. Yamada, et al., Carbon, 30, 719, 1992]. The produced membrane had an oxygen-to-nitrogen separation ratio of 4.6 and had a molecular sieve mechanism.
(5) Damle et al. used various materials and methods to perform various surface treatments on a commercial carbon membrane having a pore size of 0.2-1.0 xcexcm:  dip-coating a polymer of polyacrylonitrile (PAN), polyfurfuryl alcohol (PFA), phenol-formaldehyde resin (PF) or cellulose precursors, on the carbon membrane;  using a plasma polymerization to coat PAN on the carbon membrane;  coating a solution of a PFA resin monomer on the carbon membrane, and adding a catalyst for an in-situ polymerization;  using high temperature pyrolysis to decompose propylene into tiny carbon particles to deposit on the carbon membrane substrate [A. S. Damle at al., Gas Separation and Purification8(3), 137, 1994]. After the abovementioned processing, said membrane was subjected to high temperature carbonization in order to improve the properties of the carbon membrane. The results indicated that the permeance was reduced by all the abovementioned processing. Besides, except in-situ polymerization, the processing had no significant improvement in selectivity. Although the in-situ polymerization process slightly improved the selectivity, the transport mechanism was in Knudsen diffusion range without involving molecular sieve effect.
(6) Collins and Yin used a DC sputtering technique to coat a diamond-like carbon (DLC) on a silicon substrate, and carbonizing the coated substrate by a vacuum baking [Y. Yin and R. E. Collins, Carbon, 31 (1993) 1333]. A QCM (quartz crystal microbalance) was used to measure the absorption of benzene and 2,2-dimethylbutene in said carbon membrane. It was found that the absorption of benzene (5.2 xc3x85) was at least ten times greater than the absorption of 2,2-dimethylbutene (6.0 xc3x85). After the high temperature treatment, said diamon-like carbon membrane formed extremely fine pores on the membrane to have very conspicuous molecular sieve functions and could separate molecules of slightly different sizes. Furthermore, the porosity and the pore size distribution of said carbon membrane were affected by the sputtering conditions (e.g. composition of the gas, bias voltage of the target, etc.) and the conditions of high temperature treatments of diamond-like membrane. Therefore, the molecular sieve function could also be varied.
Meanwhile, many people have investigated silica-based molecular sieve membranes. The published results on this topic are outlined in the following:
(1) Gavalas et al. first successfully used a thermal CVD to effectively reduce the macropores of a Vycor glass substrate, by introducing SiH4 and O2 separately from both sides of the substrate in order to carry out reactions within the pores to form a molecular sieving SiO2 membrane. The selectivity of H2 to N2 reached 1000; and the permeance of H2 at 450xc2x0 C. was 10xe2x88x928 mol.mxe2x88x922.sxe2x88x921.Paxe2x88x921. They also introduced SiCl4 and O2 into the pores from the same side of said glass substrate for reaction. The reaction temperature was 600-800xc2x0 C.
(2) Yan et al. used a porous xcex1-alumina tube as a substrate, which was first subjected to a boehimite solution dip-coating, followed by pyrolysis, thereby forming an xcex3-alumina membrane on the outside of the tube [S. Yan, H. Maeda, K. Kusakabe, S. Morooka and Y. Akiyama, Ind. Eng. Chem. Res., 33 (1994) 2096]. Then, tetraethylorthosilicate (TEOS) was used as a feed in thermal CVD to deposit SiO2 membranes. The permeance of H2 at 600xc2x0 C. was 10xe2x88x927 mol.mxe2x88x922.sxe2x88x921.Paxe2x88x921, and the selectivity of H2 to N2 reached 1000.
A SiO2 membrane could only withstand a temperature up to 500xc2x0 C., and the separation of H2 often requires a higher operating temperature. Furthermore, the SiO2 membrane could only separate a mixture of small molecules (such as H2, He etc.) and other gases. Therefore, many researchers proposed the membranes containing Sixe2x80x94C or Sixe2x80x94Oxe2x80x94C structure. Such the structure could withstand a temperature up to 1200xc2x0 C. and separate gases at a higher temperature. Furthermore, the pore size could be controlled to separate a mixture of gases with similar molecule sizes (e.g. O2, N2 having a difference of 0.2 xc3x85).
(1) Tsay, Dah-Shyang et al. produced a SiC molecular sieve by adding 5% of alumina into a raw material; sintering the material into a porous tube; coating the inner wall of the tube with a SiC having a particle size of 30 nm and containing 2% of alumina; sintering said tube into an asymmetrical tube; filling the tube with a polydimethylsilane solution; and subjecting the tube to a thermal treatment, a curing and a pyrolysis. A separation membrane, which was pyrolyzed at 300xc2x0 C., had a H2 permeance of 10xe2x88x927 mol.mxe2x88x922.sxe2x88x921.Paxe2x88x921 at 200xc2x0 C., and a H2/N2 selectivity of up to 100. A separation membrane, which was pyrolyzed at 600xc2x0 C., had a H2 permeance of 5xc3x9710xe2x88x929 mol.mxe2x88x922.sxe2x88x921.Paxe2x88x921 at 200xc2x0 C., and a H2/N2 selectivity of about 40.
(2) Kusakabe and Morook et al. first used a xcex3-alumina membrane to reduce the pore size on an xcex1-alumina tubular substrate [Z. Li, K. Kusakabe and S. Morooka, J. Membrane Sci., 87 (1996) 159; Z. Li, K. Kusakabe and S. Morooka, Sep. Sci. Technol., 32 (1997) 1233]. A polycarbosilane (PC) membrane was coated thereon by pyrolysis at 350-550xc2x0 C. The membrane had a H2 permeance of 5xc3x9710xe2x88x927 mol.mxe2x88x922.sxe2x88x921.Paxe2x88x921 at 400xc2x0 C., and a H2/N2 selectivity of about 7.2. Said Sixe2x80x94Cxe2x80x94O membrane was repetitively coated three times and pyrolyzed at 950xc2x0 C., thereby increasing the H2/N2 selectivity to 18-63, but the H2 permeance was reduced to 10xe2x88x929-10xe2x88x928 mol.mxe2x88x922.sxe2x88x921.Paxe2x88x921 at 500xc2x0 C.
Therefore, there are two bottlenecks existing in the prior art methods for preparing carbon molecular sieve membranes: (1) The carbon membrane is not grown on the porous substrate. As a result, the mechanical strength of the membrane is not sufficient, and the carbon membrane is easy to crack. (2) A molecular sieve membrane grown on a porous substrate always uses a method of reducing the pore size to increase the selectivity at a cost of greatly reducing its permeance. Therefore, such a membrane has a limited applications.
A primary objective of the present invention is to provide a process for preparing a carbon molecular sieve membrane without the aforesaid drawbacks in the prior arts. The process of the present invention uses a novel technique to form a carbon molecular sieve membrane on a porous substrate, which has a high selectivity while maintaining a high permeance.
In order to achieve the abovementioned objective, a process for preparing a carbon molecular sieve membrane according to the present invention comprises the following steps:
(a) growing an amorphous carbon-based membrane on a porous substrate at a low temperature, where said membrane is able to be pyrolyzed or decomposed at a temperature higher than the growth temperature;
(b) performing a surface modification by bombarding said membrane with energized gaseous ions; and
(c) baking said surface-modified membrane at a temperature higher than the growth temperature in Step (a), so that a membrane having gas separation ability is formed.
Said amorphous carbon-based membrane comprises carbon as a major portion thereof and, optionally, other elements selected from the group consisting of Al, Si, O, N, P and F.
Said surface modification by bombarding the membrane with energized ions can be carried out with a means selected from means for generating plasma, laser or ion beams, wherein a negative bias voltage is applied to said substrate, or an ion gun or an ion implantation device can be used to accelerate ions. A suitable surface modification condition includes: bombarding said membrane with energized ions at a pressure less than 105 Pa and with an ion acceleration bias voltage less than 5000 volts, preferably 10-100 volts.
Said baking of the process of the present invention can be carried out, for example, at a pressure of 0.001 Pa-2xc3x97105 Pa, in an environment of vacuum, N2 or an inert gas such as He or Ar, and for a period of time up to 100 hours.
Preferably, the process according to the present invention further comprises repeating Steps (a), (b) and (c) one or more cycles, thereby further improving the separation effect of the membrane.
In one of the preferred embodiments of the present invention, said growth of said amorphous carbon-based membrane in Step (a) comprised using an inductively-coupled-plasma chemical vapor deposition (ICP CVD) to grow said membrane on said porous substrate. Suitable conditions for growing said membrane include: a gas phase pressure of 10xe2x88x923-100 torr; a reaction gas mixture comprising 5-100 vol. % of a carbon-containing gas such as hexamethyldisiloxane (HMDSO) or methane, and 95-0 vol. % of O2 or an oxygen-containing gas; preferably 95-5 vol. % of O2, a total flowrate of 0.5-10 sccm; RF high frequency power of 20-1000 W; and a processing time of 0.1-20 hours.
Said surface modification by bombarding said membrane with energized ions in Step (b) was carried out by means of an inductively-coupled-plasma chemical vapor deposition and applying a negative bias voltage to said substrate, wherein conditions of generating plasma include: a gas pressure of 10xe2x88x923-100 torr; a gas composition comprising 5-100 vol. % of a carbon-containing gas such as hexamethyldisiloxane (HMDSO) or methane, preferably HMDSO, 95-0 vol. % of O2 or an oxygen-containing gas, preferably O2, and 95-0 vol. % of an inert gas; a total gas flowrate of 0.5-10 sccm; RF high frequency power of 20-1000 W; and a processing time of 1-1000 seconds, preferably 3-30 seconds. Preferably, said gas composition comprises 5-100 vol. % of said carbon-containing gas, and 95-0 vol. % of O2. Preferably, said gas composition comprises 5-100 vol. % of said carbon-containing gas, and 95-0 vol. % of said inert gas.
After the surface-modified membrane being further subjected to said high temperature baking, the selectivity and the gas permeance of the resulting molecular sieve membrane can be greatly improved simultaneously. The reasons are believed to be: (1) By examining the gas permeance of a membrane which is baked often being subjected to surface modification, the increase of H2 permeance is much larger than the increase of N2 permeance. That is the voids formed by the surface modification followed by the baking only allow the passage of H2 in a large quantity while inhibiting the passage of N2. (2) The dependence of H2 permeance on the permeation temperature is opposite to that of N2. The activation energy of H2 is positive and its permeance increases with an increase in permeation temperature; while the activation energy of N2 is negative and its permeance decreases with an increase in permeation temperature. Therefore, at a higher temperature, the selectivity of H2/N2 is greatly increased. Furthermore, the duration of the surface modification and the magnitude of the negative bias voltage all need to be optimized. The surface modification duration can be adjusted in a wider range when a smaller negative bias voltage is applied. When the duration of surface modification is too long, the modification effects are not evident. Moreover, the type of gas used in the surface modification is also influential. The use of pure Ar or pure O2 in conducting the surface modification has a result not as good as that of hexamethyldisiloxane (HMDSO). However, a membrane which has been subjected to surface modification has much better separation capability than a membrane which has not been subjected to surface modification. Therefore, the composition of the gas used in the surface modification will affect the composition and the structure of the surface layer of the carbon membrane, and subsequently affect the separation performance.